1. Field of the Invention
The present invention relates to a system for the adsorption and destruction of volatile organic compounds (VOC""s) in gaseous streams which uses a single dual-function bed.
2. Related Art
Gas streams containing volatile organic compounds exhausted to the atmosphere are a source of pollution that contributes to smog and ozone at ground level. The removal of volatile organic compounds from these gas streams has been accomplished by thermal oxidation, catalytic oxidation or adsorption.
The quantity of VOC""s in the gaseous streams, such as exhaust or by-product fumes from manufacturing, printing or painting, are quite low, generally in the range of 50 to 500 parts per million. Thus the gaseous stream per se is generally too dilute to serve as a fuel source. Furthermore, because of the large volume of the gaseous stream, sending the entire stream to a flare or other destruction unit is impracticable.
The conventional practice is to first capture the VOC""s, e.g. by adsorption, then to desorb the VOC""s in a concentrated form which can then be recovered or destroyed.
Adsorption is used to collect volatile organic compounds from a gas stream at temperatures below about 180xc2x0 F. In a conventional adsorption system, two or more beds of adsorbent material are used. The gas stream is passed through one bed of adsorbent material which removes VOC""s. The other bed, which contains previously adsorbed VOC""s, is regenerated by desorbing the VOC""s in a gas stream that is heated above 180xc2x0 F. In some cases, the organic compounds are desorbed into a gas flow which is at a lower flow rate than the original gas stream. This results in a gas;stream of a higher concentration of organic compounds. This stream can subsequently be chilled to achieve solvent recovery or incinerated by thermal or catalytic oxidation to destroy the pollutants.
Adsorbent materials include activated carbon, alumina and silica gels and molecular sieves. The amount of VOC""s an adsorbent material will adsorb, depends on attributes of the particular adsorbent material such as surface area, pore size, surface composition and particle size, as well as the flow rate of the gas stream, the concentration of VOC""s and the relative humidity.
In conventional adsorption systems, a hot gas is used to desorb the VOC""s. In activated carbon adsorption systems, inert gas or steam must be used to avoid combustion of the carbon. The adsorption bed must be cooled after desorption to recover the adsorption capacity of the bed.
When a gas stream contaminated with VOC""s is passed down flow through a bed of adsorbent material, the front of the bed collects VOC""s until saturated, while the back of the bed retains adsorption capacity. In time, the entire bed will become saturated with VOC""s and the concentration of VOC""s exiting the bed will equal the concentration entering the bed. Some adsorbent materials will also adsorb water vapor from the gas stream, decreasing the relative humidity.
It is an advantage of the present invention that the separate catalytic or thermal oxidation units are either eliminated or reduced in size in the process of VOC removal and destruction from gaseous streams. It is another advantage of the present system that it may not absorption units. It is a further advantage that the present single bed system uses less energy to desorb the VOC""s. It is a feature of the present process that the adsorbed VOC""s supply a portion of the energy for their own removal. It is a particular advantage of the present invention that the system can comprise a single dual purpose bed for adsorption/oxidation. It is a further particular feature of the present invention that the adsorption/oxidation system is operated continuously with the gaseous stream.
Briefly, the present invention is a system for sequentially capturing and oxidizing gaseous VOC""s in a single bed, the process for said sequential capturing and oxidizing wherein said bed comprises an adsorbent component and an oxidation catalyst component.
In one embodiment the bed comprises a discrete adsorption component and a discrete oxidation component which are intimately mixed together in the bed. The components may be mixed such that there is a uniform distribution of the components throughout the bed or the components may be mixed in different proportions throughout the bed. In one embodiment there will be a gradation in the quantities of the components with a higher concentration of oxidation component toward the downstream end of the bed, that is in the half of the bed adjacent to the downstream end. In another embodiment the concentration of oxidation component will be higher in mid half of the bed and lower toward both the upstream and downstream ends of the bed. In a further embodiment a higher concentration of oxidation component is located toward the upstream end of the bed (the half of the bed adjacent to the upstream end).
In another embodiment the adsorbent component and oxidation component are combined on a single carrier material in an even and intimate distribution over the carrier. The carrier may be the adsorbent or a monolith.
The total oxidation component in the bed preferably ranges from 20 to 90 percent of the bed volume, either distributed evenly throughout the bed, concentrated in a portion comprising less than all of the bed or on a gradient through the bed as described. The distribution of the oxidation component may be irregular within the bed with some portion of the bed containing only adsorbent component. The balance of the bed volume not comprised of the oxidation component is preferably comprised of the adsorbent component.
The adsorbent materials include activated carbon, alumina, silica gel and molecular sieves, which may be used individually or in various admixtures.
The oxidation component may be any of the Group VB, VIB, VIII and IB metals of the Periodic Chart, particularly the noble metals, which are conventionally deposited on a suitable support such as alumina or on an alumina washcoat on a carrier.
The process (adsorption and oxidation of VOC""s) is carried out by passing a first gaseous stream containing the VOC, usually in an amount of 5 to 1000 ppm, through the treatment bed containing adsorbent component and oxidation component at a first velocity at a temperature in the bed below the temperature at which the oxidation component will function to oxidize the VOC""s, adsorbing VOC""s onto the adsorbent component, reducing the velocity of the gaseous stream below the first velocity, passing a second gaseous stream through the treatment bed at a second velocity less than that of the first velocity at a temperature in the bed to desorb said VOC""s from the adsorbent component and to cause oxidation of the VOC""s by contact with said oxidation component.
The first velocity is at a gas hourly space velocity (GHSV) in the range of 1000 to 20,000 hrxe2x88x921, preferably 5000 to 15,000 hrxe2x88x921, and the second velocity is preferably from 5 to 30 percent of the first velocity. The desorbing gaseous stream may pass through the bed in the same direction as the treatment stream or in the contra direction to the treatment stream or first in contra flow then in flow direction the same as the treatment stream. The same gaseous stream maybe used for both treatment and desorption. An advantage of first feeding the desorption gaseous stream in contra flow is the downstream oxidation component is heated so that when the flow through the bed is reversed the oxidation catalyst in the bed is already at operation temperature.